High temperature solid oxide electrolyte fuel cells (SOFC) have demonstrated the potential for high efficiency and low pollution in power generation. Successful operation of SOFCs for power generation has been limited in the past to temperatures of around 1000° C., due to insufficient electrical conduction of the electrolyte and high air electrode polarization loss at lower temperatures. U.S. Pat. Nos. 4,490,444 and 5,916,700 (Isenberg and Ruka et al. respectively) disclose one type of standard, solid oxide tubular elongated, hollow type fuel cells, which could operate at the above described relatively high temperatures. In addition to large-scale power generation, SOFCs which could operate at lower temperatures would be useful in additional applications such as auxiliary power units, residential power units and in powering light-duty vehicles.
Solid oxide electrolyte fuel cell (SOFC) generators that are constructed in such a way as not to require an absolute seal between the oxidant and the fuel streams, presently use closed ended fuel cells of circular cross section, such as shown in FIG. 1 of the drawings. Air flows inside the tubes and fuel flows outside, as shown in FIG. 2 of the drawings, where air passes through a feed tube, exits at the end of a cell and reverses flow to react with the inner fuel cell air electrode. In these cells, interconnection, electrolyte and fuel electrode layers are deposited on an extruded and sintered lanthanum manganite air electrode tube by plasma spray techniques. A lanthanum chromite interconnection is in the form of a narrow strip that runs axially over the entire active length of the air electrode tube. A yttria stabilized zirconia solid electrolyte is deposited in such a way as to almost entirely cover the air electrode tube. This yttria stabilized zirconia does not become an active electrolyte until a temperature over about 700° C. is achieved in the fuel cell. The electrolyte layer contacts or overlaps the edges of the interconnection strip leaving most of the interconnection exposed. Because the interconnection and electrolyte layers are dense, an overlap feature can provide a seal that prevents direct mixing of air and fuel gas in the air electrode.
A nickel/yttria stabilized zirconia cermet, fuel electrode anode layer is deposited in such a way as to almost entirely cover the electrolyte, but leaves a narrow margin of electrolyte between the interconnection and the fuel electrode. This margin prevents shorting of the cell. Series electrical connection between cells is accomplished by means of a structure made from nickel mesh, or, more recently, nickel foam and nickel screen, as shown in U.S. Patent Application Publication U.S. 2004/0234830 A1 (Draper et al.). The foam part of the connection becomes sintered to the interconnection while the screen part becomes sintered to the fuel electrode of the adjacent cell. Problems associated with the tubular cell are limited power density, long current path, and potential bowing after curing.
Cells of a flattened tubular, elongated, hollow, seamless parallel sided cross section, that have a number of ribs connecting the adjacent paralleled sides of the lanthanum manganite air electrode extrusion, have achieved substantially higher power density than the cylindrical cells, and are candidates to form the basic element of the next generation of SOFC generators, see FIG. 3 of the drawings. These flattened cells are described in U.S. Pat. No. 4,888,254 (Reichner) and U.S. Patent Application Publications U.S. 2007/0160886 A1 and particularly the figures in U.S. 2007/0243445 A1 (both Digiuseppe). Air flows within discrete passages that are formed between the ribs and flat sides of the air electrode. This type cell will hereinafter be referred to as “flattened” tubular, elongated, hollow type cell. They have internal gas flow channels.
These flattened tubular, elongated, hollow cells are also referred to in some instances as HPDX cells, where HPD indicates “high power density” and X indicates the number of air passages/channels. In these so called HPD cells a lanthanum chromite interconnection is preferably deposited over the entirety of one flat face of the air electrode. A yttria stabilized zirconia electrolyte covers the opposite face and the rounded edges of the air electrode so as to overlap the edges of the interconnection surface but leave most of this surface exposed. A standard nickel/yttria stabilized zirconia cermet fuel electrode covers the electrolyte except for a narrow margin of electrolyte that surrounds the interconnection. Series electrical connection between cells is accomplished by means of a nickel felt structure a flat face of which is sintered to the interconnection while the raised ribs of which are sintered to the fuel electrode face of the adjacent cell. This type cell is more efficient in generating power because of its larger active area and shorter circuit path.
Another cell geometry has been tested in which the lanthanum manganite air electrode has the geometric form of a number of integrally connected elements of triangular cross section, see FIG. 4 of the drawings. These triangular tubular, elongated, hollow cells have been referred to in some instances as Delta X cells where Delta is derived from the triangular shape of the elements and X is the number of elements. These type cells are described for example in U.S. Pat. Nos. 4,476,198; 4,874,678 (FIG. 4); U.S. Patent Application Publication U.S. 2008/0003478 A1, and International Publication No. WO 02/37589 A2 (Ackerman et al., Reichner; Greiner et al., and Thomas et al. respectively). A basic publication N. Q. Minh, in “Ceramic Fuel Cells”, J. Am. Ceramic Soc., 76 [3] 563-588, 1993 describes in detail a variety of fuel cell designs, including the tubular and triangular types, as well as materials used and accompanying reactions.
Generally, in newer triangular, tubular, elongated, hollow cross-section, so called Delta X cells, the resulting overall cross section has a flat face on one side and a multi-faceted triangular face on the other side. Air-flows within the internal discrete passages of triangular shapes where, at the end of the cell, the air can reverse flow to react with the air electrode if air feed tubes are used. In the Greiner et al. publication, above, a complicated transverse channel is used to cause reverse flow so air passes down one channel and up an adjacent one so air feed tubes can be eliminated. The fuel channels are built into multiple adjacent units of the triangular tubular type cells, and provide better fuel distribution and equal cross-section of air and fuel channels. All three designs described above, however, present problems of sealing the ends of the cell.
In the triangular tubular, elongated, hollow, so called Delta X cells, a dense lanthanum chromite interconnection covers the flat face. A yttria-stabilized zirconia electrolyte usually covers the multifaceted triangular face and overlaps the edges of the interconnection but leaves most of the interconnection exposed. A standard nickel/yttria stabilized zirconia fuel electrode usually covers most of the electrolyte but leaves a narrow margin of electrolyte between the interconnection and the fuel electrode. Series electrical connection between cells can be accomplished by means of a flat nickel felt or nickel foam panel, one face of which is sintered to the interconnection while the other face contacts the apexes of the triangular multifaceted fuel electrode face of the adjacent cell. This felt or foam also aids in shock absorption properties.
Most of these designs utilize air feed tubes, which present their own set of issues, since it is difficult to manufacture long, completely straight ones. This in turn can create problems of binding when insertion into the air feed volume of the cells is attempted.
Flattened and triangular tubular, elongated, hollow, seamless cells, FIGS. 3 and 4 operate with higher current density than current cylindrical cells and stack packing is improved. Relative to cylindrical cells, flattened and triangular tubular cells achieve less ohmic resistance, therefore cell voltage can be closer to theoretical. The triangular tubular, elongated, hollow cell, in particular, because of its thin triangular cross-sectional configuration, at open ends, poses particular difficulties in sealing and in providing transverse recirculation gas streams.
Other tubular, elongated, hollow fuel cell structures, shown in FIG. 5(a)-(g) are described by Isenberg, FIG. 7 in U.S. Pat. No. 4,728,584—“corrugated design” and by Greiner et al. FIG. 2(a)-(g)—“triangular”, “quadrilateral”, “oval”, “stepped triangle” and a “meander”; all herein considered as hollow elongated tubes. Solid oxide fuel cell generators, utilizing tubular SOFC are shown for example in U.S. Pat. No. 7,320,836 B2 (Draper et al.), showing depleted anode (fuel electrode) spent fuel gas (66) recirculation.
As described above, there is a long felt need for a fuel cell stack design that accomplishes all of: higher current density, extended fuel electrode surface per unit stack volume, better sealing of active cell ends and ease of insertion of air feed tubes, so that it provides commercialization possibilities. It is a main object of this invention to provide a single cell type that can solve all those needs. It is another object of this invention to accomplish ease of transverse spent fuel flow in a tubular cross-section design and to provide a viable commercial design.